Asynchronous IO and iowait

[dnepr]

I am trying to understand what iowait statistics really mean (or signify) when the process is using aio. Theoretically, if running aio, you wouldn't have any iowait correct? This question has been raised because we are seeing significant iowait times even though our Sybase server is using aio. These iowait times are averaging around 25-30%, but reaching as high as 50%.

How is iowait calculated in an aio environment is our question? Has anyone investigated this in depth?

Thanks, Steve

 

[bi]

Even if you are using aio, if the files of your database are poorly distributed across disks and controllers, you may see long iowait times. How are the files distributed? What kind of disks and what kind of controllers are you using?

 

[dnepr]

We are using Sybase 12.0 with raw storage on EMC Symetrix. The only jfs files would be the log files.

 

[bi]

With a setup like that, you should be getting great performance. What are your vmtune settings?